Exhaust gas decontamination system and method of controlling the same

ABSTRACT

To provide an exhaust gas purifying system and a control method therefor, capable of burning and removing PM collected at the downstream side of a DPF by utilizing HC and CO generated when performing the operation for recovering the NOx direct reduction type catalyst from a catalyst deterioration due to poisoning with sulfur.  
     The exhaust gas purifying system ( 10 ) having a NOx direct reduction type catalyst ( 3 ) for purging NOx in an exhaust gas and a DPF ( 4 ) with a catalyst for purging PM in the exhaust gas are sequentially arranged in an exhaust gas passage ( 2 ) in that order in the direction of from an upstream side to a downstream side, which further comprises an air supply system ( 5 ) for supplying air (Aa) between the NOx direct reduction type catalyst ( 3 ) and the DPF ( 4 ) with a catalyst during a operation for recovering the NOx direct reduction type catalyst ( 3 ) from a catalyst deterioration due to poisoning with sulfur by bringing the oxygen concentration in the exhaust gas to be substantially zero and raising the exhaust gas temperature.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an exhaust gas purifying system for reducingand purging NOx in exhaust gas of an internal combustion engine, andalso for collecting particulate material in exhaust gas and removingthem by burning, and relates to a control method for such a system. Moreconcretely, the invention relates to an exhaust gas purifying system anda control method for the system in which a direct reduction type NOxcatalyst is arranged upstream for purging NOx, and a DPF with anoxidation catalyst is arranged downstream for purging PM.

2. Detailed Description of the Related Art

Various studies and proposals have been offered regarding an exhaust gaspurifying system for purging particulate material (hereafter called PM)and NOx (nitrogen oxides) from exhaust gas of an automobile internalcombustion engine such as diesel engines. Concerning PM, a filter calledDPF (Diesel Particulate Filter: hereafter called DPF) has beendeveloped, and further, concerning NOx, a NOx reduction catalyst and athree-way catalyst or the like have been developed.

This DPF includes a DPF with an oxidation catalyst whose filter'ssurface is coated with the oxidation catalyst such as platinum (Pt) forcollecting PM, or a DPF with PM oxidation catalyst whose filter'ssurface is coated with a PM oxidation catalyst such as platinum and a PMoxidation catalyst such as cerium oxide (CeO₂).

The DPF with the oxidation catalyst utilizes the fact that energybarrier of PM oxidization by NO₂ is lower than that of PM oxidization byO₂ and the fact that the PM oxidization by NO₂ can be performed at alower temperature. Through the oxidation catalyst, NO in the exhaust gasis oxidized to NO₂. The collected PM is oxidized by the generated NO₂and purged.

Moreover, the DPF with the PM oxidation catalyst has the catalyst suchas cerium oxide. In the low temperature oxidation range (approximately350° C. to 450° C.), NO is oxidized to NO₂ through the oxidationcatalyst and PM is oxidized by this NO₂. In the middle temperatureoxidation range (approximately 400° C. to 600° C.), O₂ in the exhaustgas is activated through the PM oxidation catalyst and PM is directlyoxidized by the activated O₂. And in the high temperature oxidationrange (approximately 600° C. or higher) which is not lower than atemperature at which PM burns with O₂ in the exhaust gas, PM is oxidizedby O₂ in the exhaust gas.

Moreover, there are some DPFs having an oxidation catalyst such asplatinum or the like at upstream of the filter instead of coating thefilter with the oxidation catalyst. In these DPFs, NO in the exhaust gasis oxidized through the upstream oxidation catalyst, and the PMcollected in the downstream is oxidized to CO₂ by generated NO₂.

In the DPF with a oxidation catalyst and DPF having an upstreamoxidation catalyst, PM is collected and oxidized utilizing PMoxidization through the catalyst and PM oxidization by NO₂, and therebylowering the temperature so that PM can be oxidized.

However, even with these DPF with a oxidation catalyst and DPF havingupstream oxidation catalyst, it is necessary to increase the exhaust gastemperature about up to 350° C. And the exhaust gas temperature is toolow to activate the catalysts in the conditions of idling and low loadin engine operation, therefore, the above-mentioned reaction does notoccur but PM is accumulated in DPF without being oxidized. For thisreason, the operation of DPF regeneration is performed. The operation iscarried by raising exhaust gas temperature to raise the temperature ofPM up to the temperature that is not lower than the PM burningtemperature. The raising exhaust gas temperature is carried by means ofretarded injection timing, multiple stage injection, etc., or burningthe fuel supplied to the oxidation catalyst by means of post-injectionor injection in an exhaust pipe. In the operation of DPF regeneration,it is necessary to be the oxygen concentration of exhaust gas relativelyhigh and to raise the temperature of the collected PM to the PM burningtemperature in an oxidation atmosphere.

On the other hand, as one of catalysts for purging NOx, there is a NOxocclusion reduction type catalyst used for an exhaust gas purifyingsystem for an internal combustion engine proposed by the JapaneseLaid-Open Patent Publication No.2000-274279 and others. This NOxocclusion reduction type catalyst is formed with a noble metal catalystsuch as platinum and an alkaline earth such as barium (Ba) etc. on acatalyst carrier. NO in exhaust gas is oxidized to become NO₂ by thecatalytic action of the noble metal catalyst in a high oxygenconcentration atmosphere, and it is diffused into the catalyst in a formof nitric ion NO₃ ⁻ and occluded in a form of nitrate.

Then, when an air/fuel ratio becomes rich and the oxygen concentrationdecreases, the nitric ion (NO₃ ⁻) is changed to the form of NO₂ anddischarged, and NO₂ is reduced to N₂ by the reducing agents such asunburned hydrocarbon (HC), CO, and H₂ contained in the exhaust gasthrough the catalytic action. This catalytic action is able to preventNOx from being discharged into the atmospheric air.

For this purpose, the exhaust gas purifying system according to theJapanese Laid-Open Patent Publication NO.2000-274279 makes the NOxocclusion reduction type catalyst occlude NOx when an air/fuel ratio ofthe influx exhaust gas is lean, and when the NOx occlusion ability isalmost saturated, the system performs regeneration operation of thecatalyst to make the air/fuel ratio of the exhaust gas to be thetheoretical air/fuel ratio or rich, and thereby makes the catalystdischarge the NOx occluded by decreasing the oxygen concentration of theinflux exhaust gas. The catalyst reduces this discharged NOx, and thuspurifies NOx.

However, although the discharged NOx needs to be reduced by the noblemetal catalyst in this regeneration operation, a large quantity of NOxis discharged within a short time, therefore, it is difficult to reducethe whole quantity of NOx to N₂ by lefting it contact with the reducingagents and the noble metal catalyst even if a proper quantity ofreducing agents is supplied, and a part of NOx leaks, therefore, thereis the problem that the reduction of NOx has to be limited.

Further, there is another problem of sulfur poisoning that it isdifficult to maintain a high purifying rate of NOx for long hoursbecause the catalytic function deteriorates due to sulfur contained in afuel for a diesel engine.

In order to purge sulfur for recovering from the state of deteriorationcaused by the sulfur poisoning, it is necessary to raise the catalysttemperature up to 650° C. or higher, and to raise the catalysttemperature to 650° C. or higher in a diesel engine, it is necessary toraise the exhaust gas temperature to 600° C. or higher. However, even ifthe exhaust gas temperature increasing control such as intake throttleand rich burning is performed, it is actually difficult to raise thecatalyst temperature up to 650° C. only by engine control.

On the other hand, separately from the NOx occlusion reduction typecatalyst, there is a catalyst for directly reducing NOx (hereaftercalled a direct reduction type NOx catalyst) described in the PatentApplication NO.19992481 applied to the Republic of Finland andNO.20000617 applied to the Republic of Finland.

This direct reduction type NOx catalyst, as shown in FIG. 7 and FIG. 8,is the one supporting a metal M such as rhodium (Rh) and palladium (Pd)as catalyst components on a carrier T such as atype zeolite, and in ahigh oxygen concentration atmosphere as in the exhaust gas of which theair/fuel ratio of an internal combustion engine such as a diesel engineis in a lean state, the catalyst contacts NOx and reduces it to N₂, andalso this catalyst component itself is oxidized to a metal oxide MOxsuch as rhodium oxide. Since this metal M loses the ability for NOxreduction when it has completely been oxidized, it is necessary toregenerate the metal.

As shown in FIG. 8, this regeneration is performed by reducing the metaloxide MOx such as the rhodium oxide back to the metal by making themetal oxide contact with the reducing agents such as unburned HC, CO,and hydrogen H₂ in the reduction atmosphere by lowering the oxygenconcentration in the exhaust gas to almost zero percent as the air/fuelratio is the theoretical air/fuel ratio or rich state.

Moreover, this direct reduction type NOx catalyst has the advantagesthat the reaction of reducing the metal oxide MOx is speedily performedeven at lower temperature (for example, at 200° C. or higher) comparedwith other catalysts, and that the problem regarding the sulfurpoisoning is not so serious.

Further, the direct reduction type NOx catalyst is so arranged that theoxidation-reduction reaction, especially, the reducing reaction of NOxin a rich state, is promoted by mixing with cerium (Ce) which decreasesthe oxidation action of the metal M and contributes to hold NOxreduction ability as well as by providing a three-way catalyst in thelower layer. Moreover, iron (Fe) is added to the catalyst carrier toimprove a purifying rate of NOx.

However, although this type of catalyst is less sulfur-poisoned than aNOx occlusion reduction type catalyst, it deteriorates by beinggradually poisoned with sulfur in the fuel. Namely, since the sulfur inthe exhaust gas is absorbed in the iron added to the catalyst carrier ina state of SO₂, primary sulfur poisoning which inhibits the improvementof purifying performance of NOx occurs due to this iron. Further, such asecondary sulfur poisoning occurs as SO₂ discharged from the ironchanges into SO₃ in an oxidation atmosphere containing no reducing agentin a constant temperature, and as SO₃ is combined with cerium,therefore, this cerium is decreased in contribution to holding thereduction ability of NOx, and thus the purifying rate of NOx isdecreased.

However, in the direct reduction type NOx catalyst, a catalysttemperature (sulfur purging temperature) necessary for recovering thecatalytic against catalyst deterioration of this sulfur poisoning isabout 400° C. And this temperature is relatively low compared with thatfor recovering the NOx collusion reduction type catalyst which is about650° C., therefore, this temperature can easily be realized under normaldriving conditions.

When the deterioration of the direct reduction type NOx catalyst by thissulfur poisoning develops, the purifying rate of NOx is decreased due todeterioration in the reduction ability of NOx into N₂ even in a highoxygen concentration atmosphere and in a rich state of an exhaust gasair/fuel ratio. Moreover, since the NOx reduction ability soon reachesits lower limit, the regeneration operation by rich burning isfrequently required, and the fuel consumption rate becomes worsen.

Hence, in the direct reduction type NOx catalyst, the recoveringopration for sulfur deterioration by purging sulfur is necessary inaddition to the regeneration operation for reducing the oxidation metalMOx back to the metal M by contacting it with reducing agents in thereduction atmosphere. The recovering operation is performed as follows;the progress of the deterioration caused by the sulfur poisoning ismonitored, and when the deterioration reaches to some level, the sulfuris removed by raising the temperature of the catalyst to about 400° C.,the temperature not less than the one for purging sulfur. Thisrecovering operation is carried out under a low oxygen concentrationcondition in order to avoid the secondary sulfur poisoning.

However, this sulfur purging has the problem that in case of rich spikedriving for bringing the exhaust gas into a low oxygen concentration, alarge quantity of HC, CO which are unburned components is produced inthe exhaust gas and discharged outside, and this is undesirable from theviewpoint of exhaust gas purification.

SUMMARY OF THE INVENTION

The present invention is made for solving the above-mentioned problems,and the purposes of the invention are to provide an exhaust gaspurifying system capable of burning and removing PM collected on thedownstream side DPF by utilizing HC and CO generated when performing theoperation for recovering the upstream side direct reduction type NOxcatalyst from a catalyst deterioration due to poisoning with sulfur, andto provide a control method for the system.

A NOx purging system for achieving the above purposes is constituted byproviding an exhaust gas purifying system having a direct reduction typeNOx catalyst for purging NOx in an exhaust gas, and a DPF with acatalyst for purging PM in the exhaust gas arranged in an exhaust gaspassage in that order in the direction of from an upstream side to adownstream side, which further comprises an air supply system forsupplying air between the direct reduction type NOx catalyst and the DPFwith a catalyst during a operation for recovering the direct reductiontype NOx catalyst from a catalyst deterioration due to poisoning withsulfur by bringing the oxygen concentration in the exhaust gas to besubstantially zero and raising the exhaust gas temperature.

This direct reduction type NOx catalyst means a catalyst of which thecatalyst components reduce NOx (nitrogen oxides) to N₂ (nitrogen) andalso these catalyst components are oxidized when the oxygenconcentration in the exhaust gas is high, and these catalyst componentsare reduced when the oxygen concentration in the exhaust gas decreases.The direct reduction type NOx catalyst can be composed of some specialmetals such as rhodium (Rh) and palladium (Pd) carried on a catalystcarrier such as ãtype zeolite.

Further, this catalyst can be composed of cerium (Ce) for decreasingoxidation action of the catalyst component metals and letting themcontribute to holding of the NOx reducing ability. And it can be providewith a three-way catalyst having platinum or the like in the lower layerfor accelerating the oxidation-reduction reaction, especially, thereduction reaction for the NOx discharged under a rich condition.Moreover, iron can be added to the catalyst carrier for improving apurging rate of NOx.

This operation for recovering the direct reduction type NOx catalystfrom catalyst deterioration is a operation for bringing an oxygenconcentration in exhaust gas to substantially zero for avoiding thesecondary sulfur poisoning on the direct reduction type NOx catalyst,and for raising the exhaust gas temperature and thereby increasing thecatalyst temperature to sulfur purge temperature (about 400° C.) orhigher at which sulfur is exhausted. This operation can be performed bythe rich spike control such as air-intake control by an intake throttle,fuel-injection control by retarded injection, and EGR control.

Moreover, in the above-mentioned NOx purging system, the air supplysystem is arranged so as to supply a part of the air supercharged by thecompressor of a turbo-charger to a position between the direct reductiontype NOx catalyst and the DPF with a catalyst. With this arrangement,the air can be supplied by a relatively simple system.

Furthermore, as the DPF with a catalyst in the above-mentioned NOxpurging system, various kinds of DPFs having an oxidation catalyst canbe utilized. Namely, a DPF with a catalyst formed with an oxidationcatalyst carried on wall-flow type wall surfaces, and a DPF with acatalyst formed with an oxidation catalyst and a PM oxidation catalystcarried on the wall-flow type wall surfaces can be utilized. Moreover,instead of the DPF with a catalyst, a DPF with the front-arrangedoxidation catalyst can also be used.

A method for controlling NOx purging system for achieving theabove-mentioned purposes, in an exhaust gas purging system having adirect reduction type NOx catalyst for purging NOx in an exhaust gas anda DPF with a catalyst for purging PM in the exhaust gas arranged in anexhaust gas passage in that order in the direction of from an upstreamside to a downstream side, is characterized by supplying air between thedirect reduction type NOx catalyst and the DPF with a catalyst during anoperation for recovering the direct reduction type NOx catalyst from acatalyst deterioration due to poisoning with sulfur by bringing theoxygen concentration in the exhaust gas to be substantially zero andraising the exhaust gas temperature.

According to these constitution, in the case of using the directreduction NOx catalyst, when purging sulfur for recovering the directreduction type NOx catalyst from a catalyst deterioration, a largequantity of unburned components HC, CO are discharged because theexhaust gas is brought into a low oxygen state by rich spike operationto avoid the secondary sulfur poisoning. At the same time, the exhaustgas temperature is increased by the rich spike operation and the exhaustgas temperature is normally raised to 400° C. or higher at the downstream side of the direct reduction type NOx catalyst.

At this time, air is supplied to the downstream side of the directreduction type NOx catalyst, then HC and CO generated by rich spikeoperation are oxidized by the oxidation catalyst of the DPF with acatalyst at the downstream side. Because of the oxidation of HC and CO,the temperature of the PM collected in the DPF is raised and is burnedwith O₂ contained in the supplied air to be eliminated. The DPF is thusregenerated.

The exhaust gas purifying system and the control method of the systemaccording to the present invention are provided with the air supplysystem, by combining the direct reduction type NOx catalyst on theupstream side and the DPF with a catalyst on the downstream side (or theDPF with a front-arranged oxidation catalyst). In the above-mentionedsystem and method, by supplying air between the direct reduction typeNOx catalyst and the DPF with a catalyst (or the DPF with afront-arranged oxidation catalyst) at the time of the sulfur purge, theunburned HC and CO generated by the sulfur purge for the directreduction type NOx catalyst are prevented from exhausting outside; inaddition, the PM collected in the DPF with a catalyst can be burned andeliminated at the same time.

Namely, the direct reduction type NOx catalyst is selected as thecatalyst for purging NOx and the DPF with a catalyst and the DPF with afront-arranged oxidation catalyst as the DPF for purging PM and theseare arranged in the exhaust gas passage from the upstream side in order.Since the air supply system is further provided for supplying airbetween these when the operation recovering from a catalystdeterioration by the sulfur purge is performed, the unburned HC and COgenerated by the rich spike operation for the sulfur purging can beoxidized by the supplied air and purged.

At the same time, the heat generated by oxidation of the unburned HC andCO can raise the temperature of the PM collected and accumulated by theDPF with a catalyst or the DPF with a front-arranged oxidation catalystto the temperature of the re-burning of the PM or higher. Thetemperature-raised PM can be also burned with the supplied air andeliminated.

Therefore, since the regeneration operation of the DPF for purging PMcan also be performed at the time of performing the operation recoveringfrom a catalyst deterioration for the direct reduction type NOx catalystfor purging NOx, the regeneration control of the DPF can be decreased infrequency, and an increase in fuel consumption due to the DPFregeneration operation can be inhibited.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing a configuration of an engine providedwith an exhaust gas purifying system in an embodiment of the presentinvention.

FIG. 2 is an illustration showing a configuration of a means forcontrolling the exhaust gas purifying system in an embodiment of thepresent invention.

FIG. 3 is a flowchart showing an example of the exhaust gas purifyingsystem control flow in an embodiment of the present invention.

FIG. 4 is a flowchart showing an example of the catalyst regenerationcontrol flow shown at FIG. 3.

FIG. 5 is a flowchart showing an example of the control flow for theoperation recovering from a catalyst deterioration shown at FIG. 3.

FIG. 6 is a flowchart showing an example of the control flow for a DPFregeneration.

FIG. 7 is a diagrammatic view showing the reaction in the high oxygenconcentration state of the direct reduction type NOx catalyst.

FIG. 8 is a diagrammatic view showing the reaction in the low oxygenconcentration state of the direct reduction type NOx catalyst.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following, embodiments of the exhaust gas purifying system andits control method relating to the present invention will be explainedreferring to the drawings.

Firstly, the exhaust gas purifying system will be explained. As shown inFIG. 1, an exhaust gas purifying system 10 comprises a direct reductiontype NOx catalyst 3 and a DPF 4 with a catalyst arranged in an exhaustgas passage 2 of an engine main body 1 in that order in the direction offrom an upstream side to a downstream side, and further comprises an airsupply system 5 having an air supply port 5 a between the directreduction type NOx catalyst 3 and the DPF 4 with a catalyst.

As shown in FIG. 7 and FIG. 8, the direct reduction type NOx catalyst 3is formed by providing with a special metal M such as rhodium (Rh) andpalladium (Pd) on a catalyst carrier such as atype zeolite. Further,cerium (Ce) is mixed, which contributes to relaxing oxidation action ofthe metal M and holding NOx reduction ability. Moreover, a three-waycatalyst having platinum or the like is arranged in the lower layer soas to accelerate oxidation-reduction reaction, especially reductionreaction of NOx under a rich condition, and further, iron (Fe) is addedto the catalyst carrier to improve a purifying rate of NOx.

As shown in FIG. 7, in a high oxygen concentration atmosphere as in theexhaust gas having a lean air/fuel ratio of an internal combustionengine such as a diesel engine, the direct reduction type NOx catalyst 3has a property that it comes into contact with NOx to reduce NOx to N₂and also this metal M itself is oxidized to MOx such as rhodium oxide(RhOx). In addition, in the case of a reduction atmosphere having a lowoxygen concentration such as about zero % oxygen concentration in theexhaust gas as same as the air/fuel ratio is the theoretical air/fuelratio or in a rich condition as shown in FIG. 8, the oxidized metal MOxcomes into contact with the reducing agents such as unburned HC, CO, andH₂, so as to be reduced back to the original metal M such as rhodium.

The DPF 4 with a catalyst is constructed of a honeycomb filter called awall-flow type which is formed by sealing in the inlet and outlet sidesof the lots of gas passages (cells) in a staggered form. The gaspassages are partitioned in parallel by porous walls of porouscordierite or silicon carbide. Or the DPF 4 with a catalyst isconstituted of a fabric type filter laminating ceramic fibers around astainless tube with many holes.

In the case of a DPF with an oxidation catalyst, the filter isconstituted by applying an oxidation catalyst such as platinum (Pt) tothe wall surfaces of the filter. In the case of a DPF with a PMoxidation catalyst, the filter is constituted by applying an oxidationcatalyst such as platinum and a PM oxidation catalyst such as ceriumoxide (CeO₂) to the wall surfaces of the filter.

With these DPF 4 With the catalyst, unburned HC and CO can be burned inan oxidation atmosphere at 190° C.-200° C.

Moreover, the air supply system 5 is comprised of the air supply port 5a arranged just in front of the DPF 4 with the catalyst, an air supplypiping 5 c for connecting an air inlet 5 b at the downstream side of acompressor 6 a of a turbo 6 to the air supply port 5 a, and an airsupply valve 5 d arranged in the air supply piping 5 c.

A drive situation detection device 21 comprising a torque sensor and aspeed sensor for detecting the driving conditions of the engine, mainlytorque Q and engine speed Ne, is arranged. Moreover, an air/fuel ratiosensor 22 for detecting an air/fuel ratio Af is arranged at the upstreamside of the direct reduction type NOx catalyst 3 in the exhaust gaspassage 2; a catalyst temperature sensor 23 for detecting catalysttemperature Tcat is arranged in the direct reduction type NOx catalyst3; and further, a NOx sensor 24 for detecting a NOx concentration isarranged at the downstream side. Temperature sensors 25, 26 fordetecting the exhaust gas temperatures are arranged at the upstream sideof the direct reduction type NOx catalyst 3 and at the downstream sideof the DPF 4, respectively.

The exhaust gas purifying system is further comprised of a controller 50called an engine control unit (ECU) for performing general control forthe engine such as fuel injection control by receiving torque (load) Q,engine speed Ne, or the like obtained from the drive situation detectiondevice 21 or the like as input. The controller 50 is provided with ameans 200 for controlling the exhaust gas purifying system forperforming the catalyst regeneration control, the catalyst deteriorationrecovering control, the DPF regeneration control, etc. of the directreduction type NOx catalyst 3.

Moreover, in an air intake passage 7, an air cleaner 31, a compressor 6a of the turbo-charger 6, an inter-cooler 32, and an intake throttlevalve 33 are arranged. Further, as an EGR device 40, an EGR passage 41comprising an EGR valve 42 and an EGR cooler 43, and cooling-waterpiping 44 are arranged.

As shown in FIG. 2, the exhaust gas purifying system control means 200is constituted of a catalyst regeneration means 210 comprising acatalyst regeneration timing judging means 211 and a catalystregeneration control means 212, a catalyst deterioration recoveringmeans 220 comprising a sulfur purge timing judging means 221 and asulfur purge control means 222, and a DPF regeneration means comprisinga DPF regeneration timing judging means 231 and a DPF regenerationcontrol means 232.

The catalyst regeneration means 210 is a means for regeneration thedirect reduction type NOx catalyst 3, which has been contacted with NOxunder a normal driving state with high oxygen concentration in a leanstate of an air/fuel ratio of the exhaust gas and has reduced NOx to N₂and has been oxidized to a metal oxide MOx. The catalyst regenerationtiming judging means 211 judges the timing for performing this catalystregeneration. And when it judges the timing for the catalystregeneration, the catalyst regeneration control means 212 generatesexhaust gas with a zero percentage oxygen concentration of an air/fuelratio in the theoretical air/fuel ratio or a rich state, to bring theoxidation metal MOx into contact with the reducing agents such asunburned HC, CO, H₂ in an oxidation atmosphere and returns the oxidationmetal MOx to the original metal M.

Here, the normal driving state means engine operation with a torque andspeed required to the engine at the time of not performing theoperations such as regeneration operation of the direct reduction typeNOx catalyst 3, the catalyst deterioration recovering operation, theregeneration operation of the DPF 4 with the catalyst. In the normaloperation, NOx in the exhaust gas is directly reduced to N₂ through thedirect reduction type NOx catalyst 3 and purged, and PM in the exhaustgas is purged by means of the collection, burning and elimination at theDPF 4 with the catalyst.

This catalyst regeneration timing judging means 211 judges whether it isa time to regenerate the catalyst or not, based on the NOx concentrationCnox in the exhaust gas at the downstream side of the direct reductiontype NOx catalyst 3 when reducing NOx, an elapsed time of a high oxygenconcentration state, or an estimated calculation quantity of NOx to bereduced by the direct reduction type NOx catalyst when reducing NOx.

Moreover, the catalyst regeneration control means 212 is a means forlowering the oxygen concentration in the exhaust gas, namely, a meansfor performing rich spike operation of an air/fuel ratio Af of 14.7 orless. The rich spike operation is performed by any one of or acombination of following controls; fuel injection control forcontrolling the injection of fuel to be supplied to the combustionchamber of the internal combustion engine, intake quantity control forcontrolling the quantity of the intake air, or the EGR control forcontrolling the quantity of the EGR gas in the EGR device. Accordingly,the detected value Af, obtained from the above control, of the air/fuelration sensor 22 is feedback-controlled so that the value Af is within apredetermined set range.

Moreover, the fuel injection control includes a main injection timingcontrol for varying main injection timing of the fuel to be injectedinto the combustion chamber of the engine, a post-injection control forperforming post-injection after the main injection, or the like. The airintake quantity control includes an intake throttle valve control forcontrolling the opening of the intake throttle valve 33, turbo-chargerintake quantity control for controlling an intake quantity control forcontrolling an intake quantity from the compressor 6 a of theturbo-charger 6, or the like.

The catalyst deterioration recovering means 220 is comprised of thesulfur purge timing judging means 221, and the sulfur purge controlmeans 222.

The sulfur purge timing judging means 221 is a means for judging whetherto perform sulfur purge control or not. The means 221 estimates a sulfurquantity X1 to be accumulated on the direct reduction type NOx catalyst3 from fuel consumption and a sulfur concentration in the fuel, judgesto start the sulfur purge control when the cumulative sulfur quantity Xtwhich is obtained by integrating the estimated sulfur quantity X1, islarger than a judgment value X1 to start the sulfur purge. The means 221judges not to start the sulfur purge control when the value Xt issmaller than the value X1.

The sulfur purge control means 222 is a means for performing the richspike operation for lowering an oxygen concentration in the exhaust gasand also raising catalyst temperature Tcat to the temperature of thesulfur purge or above by judging it as necessary to purge sulfur whenthe cumulative sulfur quantity Xt reaches the limit X1, and therebyraises the catalyst temperature Tcat to sulfur purge temperature Tr orabove and performs the operation for recovering the direct reductiontype NOx catalyst from a catalyst deterioration due to poisoning withsulfur by purging sulfur while preventing the secondary sulfur poisoningin the rich state. Moreover, the rich spike operation in this sulfurpurge operation can be performed by any one of the fuel injectioncontrol, air intake quantity control, and EGR control or a combinationof them as the rich spike operation in the regeneration operation.

According to the present invention, the sulfur purge control 222includes the DPF regeneration control. In the DPF regeneration control,a part Aa of the supercharged air at the downstream of the compressor 6a of the turbo-charger 6 is supplied to the upstream side of the DPF 4with the catalyst by controlling the air supply valve 5 d to open. Withthis air supply, a large quantity of unburned HC and CO generated by therich spike operation in the sulfur purge control are oxidized by theoxidation catalyst of the DPF 4 with the catalyst, and further, the PMcollected by the DPF 4 with the catalyst is raised in temperature by theheat generated by the oxidation of these HC and CO, and is removed byburning with O₂ supplied by the air supply.

Namely, when purging sulfur, the exhaust gas temperature is raised bythe rich spike operation, and the catalyst temperature Tcat of thedirect reduction type NOx catalyst 3 is raised to the sulfur purgetemperature or above (about 400° C.). By supplying air at the time, theunburned HC and CO generated by the rich spike operation is burned bythe catalytic action of the oxidation catalyst of the DPF 4 with thecatalyst. The temperature of the exhaust gas flowing to the PM collectedby the DPF 4 with the catalyst can be raised further to, in general,about 500° C. Accordingly, the DPF 4 with the catalyst can beregenerated by means of removing the PM by burning.

Moreover, the DPF regeneration means 230 is a means for removing PM byburning the PM collected by the DPF 4 with the catalyst by theregeneration control with the DPF regeneration control means 232 whenthe DPF regeneration timing judging means 231 judges that the DPF isgetting clogged and the regeneration operation of DPF 4 with a catalystis necessary.

The DPF regeneration timing judging means 231 is a means for judgingregeneration timing of the DPF. The means 231 calculates the cumulativequantity of the PM by estimating the quantity of the PM to beaccumulated on the DPF 4 with the catalyst based on the operatingconditions of the engine and by integrating it. The means 231 judges thetime for regeneration of the DPF when the cumulative quantity of the PMexceeds a preset judgment value, or when a difference between thepressures before and after the DPF 4 with the catalyst or a ratio ofthem exceeds the judgment value.

Moreover, the DPF regeneration control means 232 performs theregeneration operation for the DPF 4 with the catalyst by utilizing anelectronic control fuel injection system such as a common-rail injectionsystem, and raising exhaust gas temperature by means of retardedinjection timing, multi-step injection or the like, and supplying a fuelto the oxidation catalyst applied to the filter by the post-injectionand injection in the exhaust pipe and burning it at that filter, toraise the exhaust temperature to the re-burning temperature or above.

This regeneration operation is performed in a lean burning state, or inthe state wherein the oxygen concentration of the exhaust gas flowinginto the DPF 4 with the catalyst is high by supplying air from the airsupply system 5.

Next, the exhaust gas purifying system control flow for removing NOx inthe exhaust gas by controlling the above-mentioned exhaust gas purifyingsystem 10 by the exhaust gas purifying system control means 200 will beexplained below. This control flow will be explained based on theflowcharts shown in FIG. 3 to FIG. 5 as examples.

The exhaust gas purifying system control flow shown in FIG. 3 consistsof a catalyst regeneration control at step S100, a catalystdeterioration recovering control at step S200, and a DPF regenerationcontrol at step S300. The flow is composed as a part of the entire flowfor controlling the whole engine. It is shown in FIG. 3 as the flow tobe performed synchronically with the engine control flow based upon thecall by the main engine control flow, to be interrupted with the end ofthe engine operation and returned to the main engine control flow to beended together with the control flow.

As shown in FIG. 3, when the exhaust gas purifying system control flowstarts, the catalyst regeneration control at step S100, the catalystdeterioration recovering control at step S200, and the DPF regenerationcontrol at step S300 are performed in parallel, and in case the flow hasto be ended due to the end of the engine operation or the like, aninterrupt occurs to end the control at each step and the control flowreturns to the flow, and further returns to a main engine control flowthat is not shown, to terminate this shown flow.

As shown in the catalyst regeneration control flow in FIG. 4, after thecatalyst regeneration control performs normal operation control forpurging NOx by the direct reduction type NOx catalyst 3 for apredetermined time (for example, a time equivalent to a time intervalfor judging whether or not to perform the catalyst regeneration control)at step S110, it is judged whether the direct reduction type NOxcatalyst 3 is in the regeneration start condition or not. If it is inthe regeneration start condition, the catalyst regeneration control atstep S130 is performed before the flow returns to the step S110, and ifit is not in the regeneration start condition, the flow directly returnsto the step S100, and the flow repeats this control. If this controlflow has to be ended due to ending the engine operation or the like, thetermination interrupt at step S140 occurs and the control flow returnsto the control in FIG. 3.

In the catalyst deterioration recovering control at step S200, as shownin the catalyst deterioration recovering control flow in FIG. 5, whenthe flow starts, the cumulative sulfur quantity Xt which accumulated onthe direct reduction type NOx catalyst 3 during the last engineoperation is read at step S201 from the memory.

At step S202, after performing the normal operation control for apredetermined time (for example, a time equivalent to a time intervalfor judging whether to perform the catalyst deterioration recoveringcontrol or not), a estimated quantity Xa of the sulfur accumulated bythe engine operation at the step S202 is calculated from the fuelconsumption and the sulfur concentration in the fuel, and the estimatedsulfur quantity Xa is added to the cumulative sulfur quantity Xt to makea new cumulative sulfur quantity (Xt=Xt+Xa).

At the next step S203, whether it is time to start purging sulfur or notis judged by whether the cumulative sulfur quantity Xt is larger than apredetermined purge start judgment value X1 or not. When the cumulativesulfur quantity is not larger, it is judged that it is not yet time tostart purging sulfur, and the control flow returns to the step S202.

When the cumulative sulfur quantity Xt is judged as larger than thepredetermined purge start judgment value X1 by the judgment at the stepS203, the following control at step S204-S207. The sulfur purge controlat step S204 is performed for a predetermined time. At step S205, if theexhaust gas temperature Tg1 is higher than the predetermined judgmenttemperature T1 (for example, 400° C.), at the inlet side of the directreduction type NOx catalyst, the control flow goes to step S207 afterperforming air supply at step S206, but if the exhaust gas temperatureis lower than the predetermined judgment temperature T1, the controlflow goes to the step S207 without performing air supply. Moreover,instead of using the exhaust gas temperature Tg1 for the judgment at thestep S205, the catalyst temperature Tcat can be used.

The sulfur purge control at the step S204 performs the catalystdeterioration recovering operation not only by raising the catalysttemperature Tcat to the sulfur purge temperature or above by the richspike operation, but also by decreasing the oxygen concentration in theexhaust gas to be substantially zero for preventing the generation ofSO₃ while preventing the secondary sulfur poisoning of cerium.

Moreover, by supplying air at the step S206, the unburned HC and CO thatare generated by the rich spike operation in the sulfur purge control,is oxidized and purged by means of the catalytic action of the oxidationcatalyst of the DPF 4 with the catalyst. And also the DPF 4 with thecatalyst is regenerated by raising the temperature of PM collected bythe DPF 4 with the catalyst by the heat generated from the oxidation.Then the PM is oxidized by O₂ in the supplied air Aa.

At the next step S207, the flow control calculates a discharged sulfurquantity Xs which is discharged by the sulfur purge, based on theexhaust gas quantity and the catalyst temperature Tcat (or exhausttemperature Tg1) as well as pre-inputted sulfur discharge map data,subtracting this discharged sulfur quantity Xs from the cumulativesulfur quantity Xt to obtain the new cumulative sulfur quantity Xt afterthe sulfur purge operation at the step S204. If the cumulative sulfurquantity Xt is higher than the predetermined second judgment value X2(normally it is zero) by the judgment at the step S208, the control flowreturns to the step S204 and continues the sulfur purge control untilthe cumulative sulfur quantity Xt becomes the second judgment value X2or below, and if the cumulative sulfur quantity Xt is judged as nothigher than the second judgment value X2 at the step S208, the sulfurpurge is judged as completed, and the sulfur purge control is stoppedand the control returns to the normal operation. Here, if the cumulativesulfur quantity Xt is negative, the quantity Xt is set to be zero.

Moreover, in the flow indicated at FIG. 5, the sulfur purge operation isprogrammed so as to end when the cumulative sulfur quantity Xt is judgedas the second judgment value X2 or below at the steps S207 and S208;however, the sulfur purge operation time may be calculated from thecumulative sulfur quantity Xt calculated from the fuel consumption andthe sulfur concentration in the fuel, from the exhaust gas quantity andthe catalyst temperature Tcat (or the exhaust gas temperature Tg1) atthe time of starting the sulfur purge operation, and from thepre-inputted sulfur purge operation map data, to perform the sulfurpurge control during this operation time.

Ending this step S209, the control returns to the step S202 and repeatsthe flow. When the control flow has to be terminated due to the end ofthe engine operation or the like, a termination interrupt is generatedat step S210, and the cumulative sulfur quantity Xt at the time of thetermination, namely, the cumulative sulfur quantity Xt calculated at thesteps S202 or S207 are written in the memory at step S211, and thecontrol flow then returns to the NOx purging system control flow in FIG.3 and ends.

As shown in the DPF regeneration control flow at FIG. 6, the DPFregeneration control at the step S300 performs the normal operationcontrol for collecting PM for a predetermined time (for example, a timeequivalent to the time interval for judging whether to perform the DPFregeneration control or not) at step S310; and thereafter, it is judgedat step S320 whether the DPF 4 with the catalyst is in the DPFregeneration start condition or not, and if it is in the DPFregeneration start condition, the control flow performs the DPFregeneration control at step S330 before returning to the step S310. Ifit is not in the DPF regeneration start condition, the control flowdirectly returns to the step S310, to repeat this control. When thecontrol flow has to be terminated due to the end of the engine operationor the like, a termination interrupt is generated at step S340 andreturns to the control at FIG. 3.

If the catalyst regeneration control at FIG. 4, the catalystdeterioration recovering control at FIG. 5, and the DPF regenerationcontrol at FIG. 6 return to the exhaust gas purifying system controlflow at FIG. 3 by a termination interrupt, they further return to anmain engine control flow that is not shown, and the NOx purging systemcontrol flow also ends together with the end of the main engine controlflow.

Moreover, although the above-described flow does not illustrate, any ofthe catalyst regeneration control, the catalyst purge control, the DPFregeneration control overlaps the other, any one of them is performedprior to the other according to the preset priority sequence.

According to these constitutions of the exhaust gas purifying system 10and the control method therefor, the direct reduction type NOx catalyst3 for purging NOx and the DPF 4 with the catalyst for purging PM arearranged in the exhaust gas passage in that order of from an upstreamside to a downstream side, and the air supply system 5 is arranged forsupplying air between them. The air is thus supplied to the DPF 4 withthe catalyst at the time of the operation for recovering the directreduction type NOx catalyst from a catalyst deterioration due to poisingwith sulfur by the sulfur purge, to purge by oxidizing the unburned HCand CO generated by the rich spike operation for purging sulfur, andalso the PM collected and accumulated by the DPF 4 with the catalyst canbe removed by means of burning by raising the temperature of the PM tothe PM re-burning temperature or above by the heat generated by thisoxidation.

Moreover, the DPF with the catalyst is explained as an example of a DPFso far, however, the present invention is also applicable to such a typeof DPF as an oxidation catalyst is arranged in front of the DPF insteadof the DPF with the catalyst.

In the case of the DPF with the front-arranged oxidation catalyst, thiscatalyst is constituted by coating the wall surfaces of lots of gaspassage (cells) with a noble metal catalyst depositing platinum or thelike, on alumina, zeolite, silica or the like. The passages are arrangedin a honeycomb structure formed of cordierite, silicon carbide,stainless or the like, and are penetrating from the upstream sidethrough the downstream side.

The air is supplied at the upstream side of the oxidation catalyst. Theunburned HC and CO are oxidized by the oxidation catalyst. The exhaustgas temperature is then raised by means of the heat generated by thatoxidation. The temperature of the downstream side DPF is raised by meansof raising the exhaust gas temperature. Accordingly, the PM collected bythe DPF is oxidized by O₂ in the air supplied. And the DPF is thusregenerated.

Industrial Applicability

The present invention provides an exhaust gas purifying system and acontrol method therefor, capable of removing PM collected at thedownstream side DPF by utilizing HC and CO generated at the time of theoperation for recovering the upstream side direct reduction type NOxcatalyst from catalyst deterioration due to poisoning with sulfur.

Hence, the present invention is applicable to an exhaust gas purifyingsystem combining a NOx catalyst with a DPF, and is capable ofefficiently purifying the exhaust gas from vehicles or the likeinstalling these exhaust gas purifying systems, and preventing airpollution.

1. An exhaust gas purifying system having a direct reduction type NOxcatalyst for purging NOx in an exhaust gas and a DPF with a catalyst forpurging PM in the exhaust gas arranged in an exhaust gas passage in thatorder in the direction of from an upstream side to a downstream side,which further comprises an air supply system for supplying air betweenthe direct reduction type NOx catalyst and the DPF with a catalystduring a operation for recovering the direct reduction type NOx catalystfrom a catalyst deterioration due to poisoning with sulfur by bringingthe oxygen concentration in the exhaust gas to be substantially zero andraising the exhaust gas temperature.
 2. The exhaust gas purifying systemas claimed in claim 1, wherein the air supply system supplies a part ofthe air supercharged by a compressor of a turbo-charger to a positionbetween the direct reduction type NOx catalyst and the DPF with acatalyst.
 3. The exhaust gas purifying system as claimed in claim 1,wherein the DPF with a catalyst is formed with the oxidation catalystcarried on wall-flow type wall surfaces.
 4. The exhaust gas purifyingsystem as claimed in claim 1, wherein the DPF with a catalyst is formedwith the oxidation catalyst and PM oxidation catalyst carried onwall-flow type wall surfaces.
 5. The exhaust gas purifying system asclaimed in claim 1, wherein the DPF having an oxidation catalystdisposed at the upstream side is used instead of the DPF with acatalyst.
 6. A method for controlling an exhaust gas purifying systemhaving a direct reduction type NOx catalyst for purging NOx in anexhaust gas and a DPF with a catalyst for purging PM in the exhaust gasarranged in an exhaust gas passage in that order in the direction offrom an upstream side to a downstream side, which comprises supplyingair between the direct reduction type NOx catalyst and the DPF with acatalyst during an operation for recovering the direct reduction typeNOx catalyst from a catalyst deterioration due to poisoning with sulfurby bringing the oxygen concentration in the exhaust gas to besubstantially zero and raising the exhaust gas temperature.